Composite materials in structural parts have been widely used in various fields and applications replacing materials such as metal alloys due to their superior properties of high strength-to-weight ratio, high fracture toughness and excellent corrosion resistance. Recently the amounts of composite laminates used in aerospace industry have increased dramatically, for example, the use of composite materials for the construction of airline jets such as Boeing 787 Dream-liner and Airbus A350 increases significantly and are over 50% of the whole vehicle weight, respectively. For design and safety purposes, it is crucial to maintain structural stability and rigidity during the manufacturing of the device including transportation vehicles and airplanes. Drilling of construction materials is also one of the major parts of the whole production/assembly processes. A market demand in the aerospace industry, as an example, shows that there is approximately a demand for around 50 million holes per year to be drilled utilizing automatic manufacturing process. This automation, however, should ensure the drilling quality of composite-based components while minimizing manufacturing cost and optimizing resources.
Amongst all hole-making operations for joining purpose, mechanical drilling with a drill bit is conveniently and economically adopted for producing riveted and bolted joints during assembly operations. However, the defects and damages, such as delamination, burr, microcracking, swelling, splintering and fiber pullout, are commonly visible after drilling. The delamination at the entrance and exit planes of workpiece appears to be the most critical defect, which results in lowering the bearing strength and requires additional manufacturing operation to repair for increasing its service life under fatigue loads.
Past studies, including the ones by inventors Hocheng and Tsao, shown that the thrust force is a major factor responsible for drilling-induced delamination and it mainly depends on drill materials, drill geometry and feed rate. Linear elastic fracture mechanics are employed to construct the analytical model of the drilling-induced delamination. The model correlates the delamination of composite laminates with drilling thrust force and composite material properties. One solution is to reduce the thrust force and such reducing thrust force at the drill exit in the workpiece may be adopted to avoid delamination. Hocheng and Tsao also summarized various analytical models for special drills, such as candle stick drill, saw drill, core drill and step drill, and summarize the critical thrust force models of special drill bits for delamination-free drilling of composite laminates as well as the ones of several non-traditional machining processes for composite laminates. Hocheng and Tsao realize that making the thrust force low or distributed outward from the drill center can reduce the delamination. However, such reduction of thrust force is not optimal considering the low speed of the manufacturing as a whole and the costs to the production line to accommodate such low speed drilling process during manufacturing of design parts utilizing composite materials.
Apart from the abovementioned efforts made to reduce the thrust force from drill bits, there is another solution to reduce the delamination in drilling of composite materials and that is the use of a supporting plate (consumed plate) during the drilling composite materials. Such method or practice is common in some manufacturing industries and is known as the “passive” backup utilizing a consumed plated to support the back of composite workpiece to prevent deformation leading to exit delamination. Nevertheless, such use of passive backup to support composite workpiece in drilling as a backup is not optimal, again in terms of the costs, speed and maintenance of the manufacturing process as a whole. The use of backup or consumed plate as it is always consumed and then wasted or scraped, the cost of the production is inevitably increased, especially for workpiece of large design parts. Also, operation efforts must be made during drilling process to continuously monitoring and adjusting positions of the passive backup or consumed plate such that proper backups can be provided for the composite workpiece in drilling to avoid unsupported areas of perforations or consumed areas being repeatedly utilized, which too inevitably increases the costs, speed and operation steps of the manufacturing process.
In view of the above and with regard to the drilling of composite material in the field, it is therefore optimal to provide a solution capable of reducing delamination of the drilled composite material in a layered form. In other words, there is a need for a drilling apparatus and/or providing an effective method capable of overcoming the drawbacks of the known arts to reduce delamination or propagation of cracks of composite material during drilling or after drilling thereon, such that the drilling speed of the workpiece of a composite material in a layered form may be increased with reduced or controlled delamination including propagation of cracks thereon and such that the drilling production rate and costs can be reduced significantly.